The present invention relates to an optical pickup for an optical disc player, and more particularly to a pickup having a single laser for recording and reproducing information on a disc having a different recording density.
There are optical discs such as a CD including a read only CD and a writable CD-R disc which is used as a ROM. Further, for writable optical discs having high recording densities, write-once discs and erasable discs are available. Information is recorded on the discs and reproduced with a laser beam. These discs are different from the CD in tile material of the recording surface.
For example, the write-once disc has a tellurium or bismuth recording surface on which the laser burns pits for recording. In another type of the write-once disks, the laser is focused on a recording surface coated with a selenium antimony (Sb.sub.2 Se.sub.3) thin film, or an oxide tellurium (TeOx) thin film, or a thin film of organic pigment, changing the reflectivity of the light.
The erasable disc uses as the recording surface, an amorphous alloy made of rare earth metals such as gallium, terbium, and others. In a magneto-optical recording system, the recording surface of the disc is initially magnetized to form a magnetic field in a direction perpendicular to the surface. The laser heats a predetermined area of the disc to elevate the temperature above the Curie temperature, which is about 150.degree. C., thereby reversing the direction of the magnetic field. To read the recorded information, the laser is irradiated on the recording surface so that a polarized wave front slightly rotates as a result of the Kerr effect. Thus only the polarized wave deflected by the rotation is read by a photodetector, thereby enabling to read the information.
Japanese Patent Application Laid-Open 61-220173 discloses a system for reproducing a CD having two areas the recording densities of which are different from each other. FIG. 8 shows the system. A laser beam emitted from a laser 1 is divided by a half mirror 2 into different directions. The beam in one of the directions is focused on a recording surface of an optical disc 7 through a beam splitter 3, a quarter-wave plate 4, a reflecting mirror 5, and an objective 6. The reflected beam from the disc 7 passes in reverse and is reflected by the beam splitter 3 and focused on a photodetector 9 through a cylindrical lens 8. The beam reflected on the half mirror 2 in the other direction is focused on the disc 7 through a reflecting mirror 10, a beam splitter 11, a quarter-wave plate 12, a reflecting mirror 13, and an objective 14. The reflected beam from the disc 7 is reflected on the beam splitter 11 and focused on the photodetector 9 through a cylindrical lens 15. The NA (numerical aperture) of the objective 14 is different from that of the objective 6 in accordance with the difference between the recording densities.
Japanese Patent Publication 61-49731 discloses another example of the system. FIG. 9 shows the system which comprises two lasers 16 and 17 each of which produces a laser beam having a different wavelength. The laser beam emitted from the laser 16 is focused on a disc (not shown) through a beam splitter 18, a half-wave plate 19, a beam splitter 20, a quarter-wave plate 21, and an objective (not shown). The reflected beam from the disc is reflected on the beam splitter 20 and focused on photodetector 22. The laser beam emitted from the laser 17 is reflected on the beam splitter 18 and focused on the disc in the same manner as the beam from the laser 16. The reflected beam from the disc is focused on the photodetector 22 in the same manner as hereinbefore described.
In the former system, it is necessary to provide the optical parts such as the beam splitter and the wave plate for the respective beams. Thus, the system is complicated in construction and the manufacturing cost are relatively high.
On the other hand, in the latter system, since two beams of the different wavelengths are focused on the disc, chromatic a aberration is produced. It is difficult to correct the aberration. Accordingly, it is impossible to reproduce two kinds of signals such as audio signal and a video signal recorded on the disc at the same time.
Furthermore, it is necessary to control a spot diameter of the laser beam in accordance with the recording density of the optical disc. If the disc has a high recording density, the beam must be focused on the disc at a small spot diameter. If the disc has a low recording density, the beam is focused on the disc at a large diameter. Therefore, if the beam having the large spot diameter is used for reproducing the information in the high recording density, crosstalk is produced between adjacent tracks and pits of short pitch can not be reproduced because of the large sport.
If the beam having the small spot diameter is used on the disc having the low recording density, intermodulation generates so that it is impossible to properly reproduce the information on the disc.